The transesterification process consists of the reaction of triglyceride molecules with alco­hol in the presence of a catalyst to produce glycerol and mono-alkyl fatty acid esters (Harrison et al., 2012). Biodiesel is typically transesterified using methanol, and therefore the fatty acid

alkyl esters that are produced are fatty acid methyl esters (FAME). The fatty acids are reacted with methanol to form diacyl glycerides, monacyl glycerides, and finally, fatty acid methyl esters (FAMEs) (Gong and Jiong, 2011). In this process glycerol is formed as byproduct (Figure 8.10). The transesterification process reduces the viscosity of the FAME compared to the parent oil, whereas the fatty acid composition will not be altered. FAMEs are the most prevalent alkyl esters in the current biodiesel market because of the price and availability of methanol compared to other alcohols (Knothe et al., 1997). Alcohols are the key substrates in transesterification. The commonly used alcohols are methanol, ethanol, propanol, butanol, and amyl alcohol, but methanol is widely applied in the transesterification of microalgae oils because of its low cost and physical and chemical advantages. Acid, base, or enzyme catalyzed processes may be applied in transesterification reactions (Canakci and Gerpen, 1999). The nature of the catalyst (acid/base/enzyme) influences the type of reaction. Transesterification can also be performed in the absence of catalysts using a supercritical methanol process that occurs at high temperatures (200-350 °C) and pressures (20-50 MPa). The transesterification reaction proceeds in shorter times (<5 min). Currently, this method is applied for the conversion of vegetable oils and animal fats rather than for microalgae oils (Gong and Jiong, 2011; Kusdiana and Saka, 2001).